The issue I had was black / grey triangles or blocks all over the textures / sky / ground and some areas going transparent. I originally thought it was an issue with shadows as a lot of them looked to be in areas where i would expect to see a shadow rendered but there was also small triangles making up peoples heads etc and massive long blocks flickering across huge parts of the scenery cutting people in half.
Installing the beta driver i linked to above fixed all this instantly, this driver was released about a week ago so is about the newest one available. Game runs good now, have it set on 'high' although some effects disabled. This is the first PC shooter ive played in years though so am still getting used to mouse aim again! Got a score of about 5 at the firing range so not going to be saving the planet any time soon!
Thanks for the setup help too! Is good to have a grown up discussion about this as i made the 'mistake' of mentioning it on another game site and got flamed for it!